Implementation details

Numeric/String detection is done the same way JSON::PP
does it: by using B hacks.

Boolean values are implemented as overloaded methods, that return
numbers or strings depending on the context.

_->true;
_->false;

Object-Oriented and Functional Styles

As original Underscore.js you can use Perl version in either an
object-oriented or a functional style, depending on your preference. The
following two lines of code are identical ways to double a list of
numbers.

Collection Functions (Arrays or Objects)

each_->each(list, iterator, [context])Alias: forEach
Iterates over a list of elements, yielding each in turn to an iterator
function. The iterator is bound to the context object, if one is
passed. Each invocation of iterator is called with three arguments:
(element, index, list).

reduce_->reduce(list, iterator, memo, [context])Aliases: inject, foldl
Also known as inject and foldl, reduce boils down a
list of values into a single value. Memo is the initial state
of the reduction, and each successive step of it should be returned by
iterator.

detect_->detect(list, iterator, [context])
Looks through each value in the list, returning the first one that
passes a truth test (iterator). The function returns as
soon as it finds an acceptable element, and doesn't traverse the
entire list.

sortedIndex_->sortedIndex(list, value, [iterator])
Uses a binary search to determine the index at which the valueshould be inserted into the list in order to maintain the list's
sorted order. If an iterator is passed, it will be used to compute
the sort ranking of each value.

_->sortedIndex([10, 20, 30, 40, 50], 35);
# 3

toArray_->toArray(list)
Converts the list (anything that can be iterated over), into a
real Array. Useful for transmuting the arguments object.

_->toArray({one => 1, two => 2, three => 3});
# [1, 2, 3]

size_->size(list)
Return the number of values in the list.

_->size({one => 1, two => 2, three => 3});
# 3

Array Functions

first_->first(array, [n])Alias: head
Returns the first element of an array. Passing n will
return the first n elements of the array.

_->first([5, 4, 3, 2, 1]);
# 5

rest_->rest(array, [index])Alias: tail
Returns the rest of the elements in an array. Pass an index
to return the values of the array from that index onward.

_->rest([5, 4, 3, 2, 1]);
# [4, 3, 2, 1]

last_->last(array)
Returns the last element of an array.

_->last([5, 4, 3, 2, 1]);
# 1

compact_->compact(array)
Returns a copy of the array with all falsy values removed.
In JavaScript, false, null, 0, "",
undefined and NaN are all falsy.

_->compact([0, 1, _->false, 2, '', 3]);
# [1, 2, 3]

flatten_->flatten(array)
Flattens a nested array (the nesting can be to any depth).

_->flatten([1, [2], [3, [[[4]]]]]);
# [1, 2, 3, 4];

without_->without(array, [*values])
Returns a copy of the array with all instances of the values
removed. eq is used for the equality test.

_->without([1, 2, 1, 0, 3, 1, 4], 0, 1);
# [2, 3, 4]

union_->union(*arrays)
Computes the union of the passed-in arrays: the list of unique items,
in order, that are present in one or more of the arrays.

_->union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
# [1, 2, 3, 101, 10]

intersection_->intersection(*arrays)
Computes the list of values that are the intersection of all the arrays.
Each value in the result is present in each of the arrays.

_->intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
# [1, 2]

difference_->difference(array, other)
Similar to without, but returns the values from array that
are not present in other.

_->difference([1, 2, 3, 4, 5], [5, 2, 10]);
# [1, 3, 4]

uniq_->uniq(array, [isSorted])Alias: unique
Produces a duplicate-free version of the array, using === to test
object equality. If you know in advance that the array is sorted,
passing true for isSorted will run a much faster algorithm.

_->uniq([1, 2, 1, 3, 1, 4]);
# [1, 2, 3, 4]

zip_->zip(*arrays)
Merges together the values of each of the arrays with the
values at the corresponding position. Useful when you have separate
data sources that are coordinated through matching array indexes.
If you're working with a matrix of nested arrays, zip.apply
can transpose the matrix in a similar fashion.

indexOf_->indexOf(array, value, [isSorted])
Returns the index at which value can be found in the array,
or -1 if value is not present in the array. Uses the native
indexOf function unless it's missing. If you're working with a
large array, and you know that the array is already sorted, pass true
for isSorted to use a faster binary search.

_->indexOf([1, 2, 3], 2);
# 1

lastIndexOf_->lastIndexOf(array, value)
Returns the index of the last occurrence of value in the array,
or -1 if value is not present. Uses the native lastIndexOf
function if possible.

_->lastIndexOf([1, 2, 3, 1, 2, 3], 2);
# 4

range_->range([start], stop, [step])
A function to create flexibly-numbered lists of integers, handy for
each and map loops. start, if omitted, defaults
to 0; step defaults to 1. Returns a list of integers
from start to stop, incremented (or decremented) by step,
exclusive.

Function Methods (haha, no confusion!)

bind_->bind(function, object, [*arguments])
Bind a function to an object, meaning that whenever
the function is called, the value of this will be the object.
Optionally, bind arguments to the function to pre-fill them,
also known as currying.

memoize_->memoize(function, [hashFunction])
Memoizes a given function by caching the computed result. Useful
for speeding up slow-running computations. If passed an optional
hashFunction, it will be used to compute the hash key for storing
the result, based on the arguments to the original function. The default
hashFunction just uses the first argument to the memoized function
as the key.

once_->once(function)
Creates a version of the function that can only be called one time.
Repeated calls to the modified function will have no effect, returning
the value from the original call. Useful for initialization functions,
instead of having to set a boolean flag and then check it later.

after_->after(count, function)
Creates a version of the function that will only be run after first
being called count times. Useful for grouping asynchronous responses,
where you want to be sure that all the async calls have finished, before
proceeding.

wrap_->wrap(function, wrapper)
Wraps the first function inside of the wrapper function,
passing it as the first argument. This allows the wrapper to
execute code before and after the function runs, adjust the arguments,
and execute it conditionally.

compose_->compose(*functions)
Returns the composition of a list of functions, where each function
consumes the return value of the function that follows. In math terms,
composing the functions f(), g(), and h() produces
f(g(h())).

extend_->extend(destination, *sources)
Copy all of the properties in the source objects over to the
destination object. It's in-order, to the last source will override
properties of the same name in previous arguments.

_->extend({name => 'moe'}, {age => 50});
# {name => 'moe', age => 50}

defaults_->defaults(object, *defaults)
Fill in missing properties in object with default values from the
defaults objects. As soon as the property is filled, further defaults
will have no effect.

clone_->clone(object)
Create a shallow-copied clone of the object. Any nested objects
or arrays will be copied by reference, not duplicated.

_->clone({name => 'moe'});
# {name => 'moe'};

tap_->tap(object, interceptor)
Invokes interceptor with the object, and then returns
object. The primary purpose of this method is to "tap into" a
method chain, in order to perform operations on intermediate results
within the chain.

mixin_->mixin(object)
Allows you to extend Underscore with your own utility functions. Pass
a hash of {name => function} definitions to have your functions
added to the Underscore object, as well as the OOP wrapper.

uniqueId_->uniqueId([prefix])
Generate a globally-unique id. If prefix is passed, the id will be
appended to it.

_->uniqueId('contact_');
# 'contact_104'

template_->template(templateString, [context])
Compiles templates into functions that can be evaluated
for rendering. Useful for rendering complicated bits of HTML from JSON
data sources. Template functions can both interpolate variables, using
<%= … %>, as well as execute arbitrary Perl code, with
<% … %>. When you evaluate a template function, pass in a
context object that has properties corresponding to the template's free
variables. If you're writing a one-off, you can pass the context
object as the second parameter to template in order to render
immediately instead of returning a template function.